The present invention relates to an image transferring device for a copier, printer or similar electrophotographic image forming apparatus and, more particularly, to a positional relation between a transfer bias section and a discharge section with respect to a sheet and control over the transfer bias in an image transferring device of the type transferring an image from an image carrier to a transfer belt while transporting the sheet and causing it to electrostatically adhere to the belt. The present invention is also concerned with a separating device capable of surely separating a transfer medium in the form of a sheet with no regard to the environment.
It is a common practice with an image forming apparatus to use an image transferring device of the type electrostatically transferring a toner image formed on an image carrier, or photoconductive element, to a sheet carried on a transfer belt to which an electric field opposite in polarity to the toner image is applied, and a separating device for separating the sheet from the photoconductive element. The devices of the type described usually includes an arrangement for applying a transfer bias to the transfer belt. For example, an electrode member is connected to a high-tension power source and held in contact with the rear of the belt at an image transfer position. Such an arrangement, or so-called contact type transfer and separation arrangement, is advantageous over one which relies on a corona charger since it does not produce harmful ozone and can operate with a low voltage. The transfer belt is sometimes replaced with a transfer roller. The contact type transfer and separation system has been proposed in various forms, as disclosed in Japanese Patent Laid-Open Publication Nos. 123385/1990, 123386/1990, 287380/1990, and 287381/1990 by way of example.
In addition to transferring a toner image from the photoconductive element to the sheet, the devices for transferring an image and separating a sheet described above deposit a polarized charge on the sheet by the transfer bias so as to cause the sheet to electrostatically adhere to the belt. Therefore, as the belt is moved, the sheet can be transported by the belt and separated from the belt due to the electrostatic adhesion.
However, when the sheet is caused to electrostatically adhere to the belt, it has to be separated from the belt after image transfer. For the separation of the sheet, use may be made of a transfer belt having a resistance of 10.sup.10 .multidot..OMEGA..multidot.cm to 10.sup.13 .multidot..OMEGA..cm, and a discharge member located downstream of an image transfer position with respect to an intended direction of movement of the belt for dissipating the charge of the belt, as disclosed in Japanese Patent Laid-Open Publication No. 83762/1988 by way of example. The discharge member reduces or cancels the charge of the sheet to promote easy separation of the sheet. Regarding the discharge of the belt, Japanese Patent Laid-Open Publication No. 96838/1978, for example, teaches an arrangement which uses a transfer belt having a resistance of 10.sup.8 .OMEGA..cm to 10.sup.13 .OMEGA..cm and, in the event of continuously transferring images from a plurality of photoconductive elements to a sheet carried on the belt, dissipates a charge of the belt deposited by a discharge ascribable to the separation of the sheet from one photoconductive element before the belt faces the next element.
On the other hand, when the transfer bias is maintained constant, a current to flow to the photoconductive element changes relative to the bias set at the transfer belt side due to changes in temperature, humidity and other environmental conditions. For example, in a high temperature and high humidity environment, an excessive current is apt to flow to the photoconductive element since the belt and sheet absorb moisture to lower their resistances. This increases the charge deposited on the photoconductive element and often causes the sheet to wrap around the element. In the opposite environment, the transfer of a toner image becomes defective. In the light of this, use may be made of control circuitry having a controller for controlling the output current of a high-tension power source and to which a roller which supports the belt is connected, as taught in, for example, Japanese Patent Laid-Open Publication No. 231274/1991. The control circuitry detects the output current of the power source by the support roller via the belt and controls the output current in matching relation to a feedback current flowing through the support roller. With such control circuitry, it is possible to maintain the current to flow to the drum constant and thereby prevent the sheet from wrapping around the drum while eliminating defective image transfer.
However, simply selecting an electric characteristic with regard to the belt is not satisfactory when the transfer bias or the discharging operation is to be set as stated above. Particularly, it is necessary to eliminate the wrapping of the sheet, defective image transfer and incomplete sheet separation by adequately positioning the constituents of the image transfer device relative to each other and selecting adequate materials at the actual design stage. Moreover, for the control of the surface potential of the sheet via the belt, not only changes in environment but also other factors, e.g., changes in surface potential ascribable to changes in resistance which are in turn ascribable to irregularities in the quality of belts particular to the production line and the size of an image have to be taken into account. Should such changes be neglected, the amount of charge for setting up an electric field required for image transfer would change. This would not only degrade the quality of an image but also aggravate the defective sheet separation.
On the other hand, there is available a copier or similar electrophotographic image forming apparatus of the type including a carrier for carrying a toner image transferred thereto at a transfer position and transporting it while being rotated. In this type of apparatus, a toner image formed on a photoconductive element is transferred to a belt at a first transfer position. As the belt is rotated to transport the toner image to a second transfer position, the toner image is transferred from the belt to a sheet. At a position upstream of the first transfer position, a transfer potential is applied to the belt to transfer the toner image from the photoconductive element to the belt.
The contact type image transfer and sheet separation system is advantageous over the corona type system in that it reduces ozone and requires only a low power source voltage, as discussed above. However, the problem with the transfer belt is that the adequate bias voltage to be applied from the power source to the belt changes due to various causes including irregularities in the resistance of the belt, varying ambient conditions, kind of sheets, and area of a toner image. This prevents the toner image from being surely transferred from the belt to the sheet. Specifically, the amount of charge deposited on the belt by the bias potential from the power source deviates from one required to effect desirable image transfer due to irregularities in the resistance of the belt ascribable to the production line, changes in the resistance ascribable to the varying ambient conditions, changes in the material and thickness of sheets, etc. More specifically, when the amount of charge required to effect desirable image transfer is deposited on a transfer belt, discharge does not occur in a pretransfer region upstream of the nip portion between the photoconductive element and the belt. In this condition, a toner charged to positive polarity, for example, is transferred to the sheet carried on the belt in a transfer region. In this case, a bias potential is applied from a power source to the belt. When the actual amount of charge on the belt is deviated from the expected one due to the above-stated reasons, discharge occurs in the pretransfer region. This causes a negative charge to deposit on the toner and thereby charges the front and the rear of the belt to positive polarity and negative polarity, respectively. As a result, despite that the bias potential from the power source is adequate, the toner is prevented from being transferred from the photoconductive element to the sheet, resulting in the local omission of an image on the sheet.
Further, the transfer belt not only transfers the toner image from the photoconductive element to a sheet or similar transfer medium, but also separates the sheet from the element by electrostatically retaining it thereon. However, the problem is that the separation of the sheet from the photoconductive element depends on the ambient conditions. Particularly, when the water content of the sheet increases in a hot and humid environment, it is likely that the sheet is adhered to the photoconductive element and not to the belt and cannot be separated from the element. Should the sheet be forcibly separated from the photoconductive element by a pawl or similar implementation, it would scratched or creased to degrade the image quality.
In the electrophotographic image forming apparatus, a transfer potential is applied to the belt at a position upstream of the first transfer position so as to transfer the toner image from the photoconductive element to the belt. This brings about a problem that the toner flies toward the belt at a position upstream of the first transfer position, thickening lines, blurring characters, reducing sharpness or otherwise degrading images.